This invention relates to reverse osmosis liquid purification systems, and, more particularly, to establishing the operating and maintenance procedures for such a system.
Reverse osmosis is used to purify liquids that contain dissolved and undissolved impurities, in a variety of industrial, commercial, and home applications. The liquid to be treated is passed over a reverse osmosis membrane in a cross flow manner. Some of the liquid, termed the "permeate", passes through the reverse osmosis membrane and is collected as the purified liquid that is the objective of the procedure. The remainder of the liquid, now having a higher concentration of the impurities and termed the "concentrate", is discarded or futher processed. The permeate has a lower concentration of impurities than does the concentrate.
Although much of the impurity material leaves the reverse osmosis unit in the concentrate, some remains adhered to the reverse osmosis membrane as a colloidal solid. Undissolved impurities include substances found widely in process liquids such as clays, silica, iron and aluminum hydroxides, and organic debris, and also substances that may be peculiar to a particular application such as paint pigments, proteins, high-molecular-weight alcohols, and bacterial and yeast cells. Flocculents may be added to the impure liquid upstream of the reverse osmosis unit, and prior to its passing through filters or clarifiers, to cause colloidal solids to form, so that the impurities in the colloidal solids may be removed by the filters or clarifiers.
The colloidal solids adhering to the reverse osmosis membrane gradually build up over time, eventually forming layers of colloidal solid material that interfere with the operation of the reverse osmosis membrane and decrease its efficiency. This gradual reduction of operational efficiency is termed "colloidal fouling". The result of colloidal fouling is that the permeate flow rate is gradually reduced and eventually becomes unacceptably low.
When the mass of colloidal solids on the reverse osmosis membrane becomes so thick and dense that the permeate flow is reduced to an unacceptably low level, the reverse osmosis unit is removed from service and cleaned. Such units are typically designed as integral tubular housings containing the reverse osmosis membranes in a rolled-up form, that can be removed from the reverse osmosis system, replaced by a spare unit so that the system continues to operate, and sent to a cleaning operation. Alternatively, they may be removed from service and cleaned in place. In the cleaning operation, the colloidal solids are removed and the unit is otherwise reconditioned. The cost of each reverse osmosis unit is such that cleaning is economically attractive as compared with installing a new unit each time the colloidal fouling becomes excessive.
The incidence of colloidal fouling depends upon the nature and impurity content of the liquid that is being passed through the reverse osmosis system. Typically, the more contaminated is the liquid with impurities that form colloidal deposits, the more often is cleaning required. It is desired to increase the time between cleanings as much as possible, and to know the typical time between cleanings as precisely as possible to permit the designing of the reverse osmosis system with as little overdesign as possible, to reduce down time for maintenance, and to minimize the number of spare reverse osmosis units that must be kept on hand.
An important advance in reverse osmosis systems has been the development of antifouling compositions that are introduced into the feed liquid, typically a contaminated water, upstream of the reverse osmosis unit. The antifouling compositions, added in amounts of parts per million to the contaminated liquid, chemically alter the nature of the colloidal solids so that they are less prone to adhere to the reverse osmosis membranes. That is, the treated colloidal solids tend to pass out of the system in the concentrate rather than stick to the reverse osmosis membrane and cause colloidal fouling.
The amount of the antifouling compound added to the liquid to be treated must be controlled precisely. If either too little or too much is added, the rate of colloidal fouling is greater than if an optimum amount is added. Moreover, if too much antifouling compound is added, the excess over the optimum amount is wasted. Since reverse osmosis units are often used to process very large amounts of liquid, such as drinking water supplies, small amounts of wasted antifouling compound (as measured in parts per million) can become expensive.
There are two principal issues that must be resolved as to the proper concentration of an operable antifouling compound to be added to the liquid being treated. First, the optimum concentration must be established. The optimum concentration varies with each type of feed liquid, and no generalization or a priori prediction technique is known. Second, with this optimum concentration established, the frequency of cleaning of the reverse osmosis units must be determined.
In current practice, the optimum concentration of the antifouling compound and the cleaning frequency are established by a trial-and-error process. Various amounts of the antifouling compound are added to the liquid being treated, the reverse osmosis system is operated for a period of months, and the reverse osmosis units are removed from service and evaluated. As an aid in making the determination, the feed liquid is usually characterized as to the amount of undissolved material by a standardized silt density index ("SDI") test known in the industry. In the SDI test, a fixed quantity of the feed liquid is passed through a standard absolute 0.45 micrometer filter and the amount of particulate captured by the filter is determined by the weight gain of the filter during the test. In practice, it is observed that the SDI test does not accurately characterize the colloidal fouling tendency of reverse osmosis membranes and units.
Consequently, at the present time the determination of optimum additions of colloidal antifouling compounds and the maintenance schedules of reverse osmosis membranes and units remains largely a matter of trial-and-error that requires months of non-optimal use of the reverse osmosis process to find the optimum operating and maintenance parameters. If there is a change in the nature of the feed liquid or of the colloidal antifouling composition, the trial-and-error process must be repeated (although the prior conditions usually provide some guidelines). There is therefore a need for an improved technique for determining the required addition of antifouling compound and the associated maintenance period of the reverse osmosis unit. The present invention fulfills this need, and further provides related advantages.